Anyone who has ever viewed an image on a television set, a computer monitor, a projection screen or other projection surface, and the like, where one corner of the image was out of alignment has experienced a form distortion, namely, a geometric distortion called “keystoning” or “keystone” geometric distortion. Keystoning is where an image is out of alignment, conventionally at one or more of the corners, of the image producing a trapezoidal or trapezoidal-like shape.
Computer monitors and televisions sets conventionally come with mechanisms for manually adjusting a display screen image, not only at the corners but also top and bottom side adjustments and left and right side adjustments. However, these adjustments conventionally require using multiple adjustment modes, where an adjustment in one mode may affect another adjustment in another mode. This leads to a time consuming iterative approach to bring a display image into alignment.
Iterative adjustment for image projectors conventionally involves moving the projector itself. Thus, a projector often occupies a central point in a room, where leg adjustment screws are used to adjust to a support surface on which the projector is located. The projector lens is thus aligned to be at least substantially parallel with a projection surface. Adjusting position of a projector conventionally is an iterative approach where one adjustment affects another. An additional annoyance is having a projector in the way of a presenter and attendees, as the projector conventionally is centered to the projected image to minimize distortion. If a projector is not on-axis with respect to a projection surface (i.e., the direction of the projection is not perpendicular to the screen), keystoning may result.
Modern digital projectors have keystone correction allowing for “squaring up” a projected image when projecting off-axis. However, most of these corrections are optical adjustments and not image adjustments. Accordingly, off-axis angle is limited owing to manufacturing cost of more exotic lenses. Still others have suggested warping of an image to compensate for keystoning. However, such approaches conventionally involve addition of a camera to sense a displayed image or require data input to determine position of a displayed image. Moreover, such systems involve special circuitry and/or complex software to warp an image to correct for keystoning.
Furthermore, projecting an image off-axis to a screen or imaging surface can have effects other than keystoning, such as anisotropic or aspect ratio geometric distortion. An example of anisotropic geometric distortion is text that progressively expands vertically, namely, an unequal image display above and below a horizontal optical axis. Anisotropic distortion limits use of off-axis projection, even though it may be desirable to position a projector off-axis in order to have more distance from a projection surface to project a larger image.
Accordingly, it would be desirable and useful to provide method and apparatus for display image adjustment that is more intuitive for a user and costs less than optical adjustment or prior image warp techniques. Additionally, it would be desirable and useful if such method and apparatus enhanced ability for off-axis projection.